Dimethyl ether (DME) can be produced by one-step method and two-step method. The one-step method refers to one-step synthesis of DME from syngas, and the two-step method refers to synthesis of methanol from syngas, and then preparation of DME via dehydration.
The two-step method is carried out via two steps, i.e. synthesizing methanol from syngas, and then dehydrating methanol with the catalysis of an acid to prepare DME. The two-step method for the DME synthesis is a main process for producing DME home and abroad. Said two-step method uses fine methanol as feedstock, and has the advantages of less by-products of the dehydration reaction, high purity of dimethyl ether, mature technique, wide adaptability of the device, and simple post-treatment. Said two-step method can be directly used in a methanol factory, or other non-methanol factory having established public utilities. Generally, ZSM-5 molecular sieve comprising γAl2O3/SiO2 is used home and abroad as the dehydration catalyst. The dehydration temperature is controlled at 280-340° C. under a pressure of 0.5-0.8 MPa. The single-pass conversion of methanol is from 70 to 85%; and the DME selectivity is higher than 98%.
CN1180064A discloses a method of producing DME. Said method uses methanol as feedstock. The dehydration reaction is conducted at a relative low temperature (100-125° C.) under a normal pressure (0-0.05 MPa, gauge) in the presence of a fresh catalyst to produce a DME gas.
CN1368493A discloses a method of producing DME by methanol catalytic dehydration. It relates to a method of producing DME by a methanol catalytic dehydration, wherein said dehydration is conducted in the presence of a solid acid catalyst containing SO42+. The SO42+ content in the catalyst is preferably 2-25 wt %. The preferred catalyst support is selected from γ-Al2O3, η-Al2O3 and SiO2.
CN1301686A discloses a method of producing DME by methanol dehydration. In said method, a catalyst, which uses kaolin as staring material and is modified with sulfuric acid, is used in the methanol dehydration to produce DME.
US2004/0034255A1 discloses a method of producing DME by catalyzing the gas phase methanol dehydration with an active alumina. Said active alumina has a pore diameter of 2.5-8.0 nm, wherein the Na2O content is below 0.07%.
The above mentioned methods primarily concern producing DME by catalyzing the methanol dehydration with composite solid acids, acid-modified kaolin, active alumina, and the like. Moreover, these methods mainly use fixed bed reactors to produce DME for fine chemicals and have a small production scale and a higher production cost.
In addition, the methanol dehydration is a strong exothermal reaction, and an adiabatic or continuously-heat-exchanging fixed bed reactor is generally used as the reactor, therefore, it is difficult to control the fixed bed temperature.
At present, the technical process of the catalytic dehydration of methanol in a gas phase to produce DME is generally as follows: the methanol feedstock is heated via a vaporizer or a vaporizing column and all vaporized, and then is sent to a reactor to conduct the reaction; the reaction product from the reactor is condensed, and then sent to a DME rectification column to conduct the rectifying separation; the DME product is obtained from the DME rectification column top, and a mixture of methanol and water is discharged from the DME rectification column bottom and enters a methanol recovery column to conduct the rectifying separation; methanol obtained from the methanol recovery column top is sent back to a methanol buffer tank to mix with the methanol feedstock and re-vaporize; and waste water from the methanol recovery column bottom is discharged out of the system.
U.S. Pat. No. 5,037,511 discloses a method of producing pure DME from methanol. In said method, methanol is vaporized by heat-exchange, and is subjected to the catalytic dehydration reaction in an adiabatic fixed bed reactor. The dehydrated reaction product enters a DME rectification column to conduct the rectification to produce a DME product of high purity. Noncondensable gas from the column top is washed with the methanol feedstock and then emitted. Due to the absence of heat collector in the reactor, the methanol dehydration reaction has a wide reaction temperature and a low methanol conversion, and produces more by-products. The rectification column is provided with a base-washing line and a water-washing line. The process is quite complex.
Chinese Patent ZL 95113028.5 discloses a method of producing DME from methanol. Its object is to provide a DME production process which can use a raw methanol as feedstock. The methanol feedstock has a concentration of 72% or more. The raw methanol feedstock is firstly sent to a vaporization-separation column to remove high boiling point materials and impurity, and then subjected to the catalytic dehydration reaction in the presence of a complex solid acid catalyst in a multistage-quenching-type reactor. Because methanol vapor enters the multistage-quenching-type reactor by stages, the gas which is subjected to the dehydration reaction in the former stage has a higher temperature and can be cooled by the methanol vapor with a lower temperature from the latter stage, so as to avoid the temperature rise and is in favor of increasing the conversion. However, since the methanol vapor has a low heat capacity, the methanol vapor has a limited function as the cooling medium. The reaction temperature is relatively high in the quenching-type reactor. The reaction temperature range is still relative wide so as to produce more by-products. Therefore, said method has a low single-pass conversion and a decreased product yield, and is not suitable for a large scale industrial production. The dehydrated product enters a packed DME rectification column to conduct the rectification so that a DME product with a purity of 90-99.99% can be produced. Noncondensable gas from the DME rectification column top enters the absorbing column to be washed. Noncondensable gas such as H2 and CH4 is emitted from the absorbing column top. The absorbing liquid used in the absorbing column is not described in details.
For the purpose of decreasing the massive energy consumption required by vaporizing the methanol feedstock and saving the device investment, Chinese patent 200410022020.5 proposes another method for producing DME. In said method, a methanol feedstock vaporizing column and a methanol recovery column are combined into a vaporizing-stripping column. The methanol feedstock with a methanol content of 70-99.99% enters the top of the vaporizing-stripping column to be vaporized in said column. The DME rectification column bottom liquid enters the middle part of the vaporizing-stripping column to separate methanol and water in said column. Said vaporizing-stripping column has both a function of vaporizing the methanol feedstock and a function of separating and recovering the aqueous methanol solution. It can not only dispense with the investment for the methanol recovery column and the auxiliary equipments, but also sharply reduce the energy consumption for recovering methanol from the mixed liquid from the DME rectification column bottom. However, in said method, all of the methanol feedstock enters the vaporizing column, the liquid phase load is too heavy, and it is difficult for the practical operation to guarantee the methanol concentration in the column bottom waste water to be reduced to a low level. Therefore, another stripping column is generally required to treat the waste water containing a little methanol coming from the vaporizing-stripping column. At the same time, due to the heavy liquid phase load, the vaporizing-stripping column should be provided with a large column diameter, and the investment consequently increases. Especially in case of a low concentration of the methanol feedstock, the concentration of the gas-phase methanol in the column top can not be adjusted, and it contains a large quantity of water; therefore, the reaction equilibrium conversion decreases so as to reduce the single-pass yield of the product.
In order to overcome the shortcoming of the heavy load on the vaporizing-stripping column of Chinese Patent ZL 200410022020.5, CN1919819A discloses a novel DME production process, wherein a part of the methanol material enters the methanol rectification recovery column top as a reflux liquid of the methanol rectification recovery column, and the other part enters the methanol pre-heater to heat-exchange with a gas mixture formed via reaction, enters the methanol superheater together with the methanol rectification recovery column top gas, and then enters the cooling-tube reactor to react. Said process can flexibly adjust the methanol vaporization depending on the different methanol feedstock, and reduce the heat load on the methanol rectification recovery column. However, since said process still adopts an adiabatic fixed bed reactor, the reaction temperature is relative high and more by-products are produced.
CN1830934A discloses a method for producing DME from methanol. Said method uses a fixed bed reactor having a built-in heat exchanging calandria. A methanol gas is used to remove a portion of reaction heat in the heat exchanging calandria. This solves the problem of a relative high reaction temperature in the fixed bed reactor to some extent. The methanol feedstock firstly enters an alcohol washing column to wash off the noncondensable tail gas coming from the DME rectification column as the reaction by-product, and then enters a methanol column to vaporize. The vaporized methanol enters the built-in heat exchanging calandria of the reactor to be superheated, and then enters the catalyst bed from the reactor top to react. The reaction product, after heat-exchanging, enters the DME rectification column in a gas phase to conduct the rectification. Said method utilizes a part of reaction heat, decreases the reaction temperature rise and reduces the reaction by-product. However, because the heat-collecting medium is the gas phase methanol, the removal of heat only by the sensible heat of the gas has a limited effect. Thus, the effect of controlling the reactor temperature and the energy consumption reduction are not remarkable.
In summary, one feature of the existing DME production methods lies in the methanol feedstock, including the methanol recovered by the methanol recovery column. The heat for its vaporization is always from the vaporizer, the vaporizing column, the methanol recovery column or the reaction product, and not directly from the methanol dehydration reaction. Therefore, the reaction has a high temperature rise and produces more by-products. On the other hand, in order to control the methanol dehydration reaction temperature in the reactor, the existing methods use the gas phase methanol as cooling medium, in a direct heat-exchanging manner for example in which the methanol gas is injected into the quenching-type reactor, or in an indirect heat-exchanging manner such as that of the built-in heat exchanging calandria. However, because the heat-collecting medium is the gas phase methanol, the removal of heat only by the sensible heat of the gas has a limited effect. Thus, the effect of controlling the reactor temperature and the energy consumption reduction are not remarkable.
Another feature of the existing DME production methods lies in that the methanol feedstock is used as the washing liquid or the absorbing liquid in the alcohol washing column or the absorbing column Noncondensable gases emitted from the top of the gas-liquid separator or the top of the DME rectification column entrain a small amount of methanol and DME, which are absorbed with the methanol feedstock in the existing methods. However, the solubility of DME in methanol is low, and therefore a large quantity of the methanol feedstock is required to be sent to the alcohol washing column or the absorbing column, and the absorbing efficiency is low. When the DME production is scaled up, methanol and DME entrained in a large quantity of noncondensable gases in the reaction product require a large quantity of methanol for washing and absorbing. This results in a heavy liquid phase load on the alcohol washing column and the absorbing column, a large column diameter, and an increased equipment investment.